1. Field of the Invention
The present invention relates to an improved igniter for solid fuel rocket motors and has particular utility in high performance solid fuel rocket motors having full, head-end-web propellants.
2. Description of the Prior Art
The conventional igniter for large solid fuel rocket motors is a miniature rocket motor that is known in the art as a "pyrogen" igniter. Typically, a pyrogen igniter is mounted in the forward end of a motor through a hole in the propellant. In some high-performance rocket motors, however, a pyrogen igniter is impractical to use because of the full, head-end-web of the propellant in the forward end of the motor. The head-end-web propellant grain design has the primary advantage of providing a higher mass fraction.
Full, head-end-web solid propellant rocket motors are now being developed which have the capability of being offloaded, that is, having some of the propellant grain removed to meet specific total impulse requirements. Such offloading is achieved by machining out propellant from the aft end of the motor. As the propellant is removed, the internal free volume of the motor increases proportionately. The larger free volume makes ignition of the motor more difficult. One way to improve the ignition process is to mount the igniter to the aft surface of the motor propellant grain. The source of heat for ignition thus remains as close as possible to the surface being ignited, regardless of the degree of offload.
Mounting the igniter to the motor propellant grain introduces a problem. This is because, upon ignition and burning of the propellant grain at the aft end of the motor, the support for the igniter erodes. Such erosion tends to cause the igniter to become detached from the propellant grain with possible resulting damage to the rocket motor nozzle and/or the propellant. In order to avoid such damage, the igniter must be substantially consumed before becoming detached from the propellant grain. Additionally, the igniter must perform its intended function of igniting the propellant grain before being consumed. A further requirement is that the igniter must exhaust or project its output across the surface of the motor propellant surface regardless of the percent of offload.
In a form of igniter for high-performance solid fuel rocket motors proposed in the prior art, as disclosed in the copending application for patent of C. Max White bearing Ser. No. 463,102 filed Feb. 2, 1983, now U.S. Pat. No. 4,498,291, and assigned to Thiokol Corporation, the assignee of the present invention, there is provided a consumable wafer-like igniter comprising cylindrical hat-shaped housing with a projecting rim or flange. The flange is bonded to the surface of the motor propellant grain, and being so bonded, forms the igniter chamber. A problem with this igniter is that the internal or chamber pressure of the igniter tries to force the igniter off the surface of the motor propellant grain. Thus, the allowable igniter internal pressure is dependent upon the integrity of the bond between the igniter flange and the surface of the propellant grain. Another problem with this igniter stems from the internal volume thereof being fixed and difficult to adjust and is manifested as an instability, known in the art as the "L*" instability. This is a phenomenon that tends to occur when the igniter is fired in a very large vacuum chamber. The instability is encountered in this situation when the free volume of the igniter chamber is too small. Specifically, when the igniter chamber is too small, minor variations in igniter chamber volume can cause an oscillation in the rate of burning of the igniter propellant and resulting extinguishment thereof.
Another form of igniter for high-performance solid fuel rocket motors is disclosed in my copending application Ser. No. 453,318, filed Dec. 27, 1982, now U.S. Pat. No. 4,503,773, and assigned to Thiokol Corporation. This igniter comprises a closed pressure vessel that is attached to a lined cutout or cavity in the surface of the motor propellant grain. It does not depend upon a bond between the surface of the motor propellant grain and the igniter to hold the igniter internal pressure. That is to say, the thrust of the igniter is neutral so that it neither applies stresses to the motor propellant nor tries to eject the igniter from the cavity. The liner for the cavity may be machinable with the aft end of the motor propellant grain so that propellant grain cut back can readily be achieved to accommodate, as required, a change in mission of the motor. The need for a cavity and a liner therefor for attaching the igniter to the motor propellant grain introduces an aspect of complexity and cost that indicates a need for further improvements in high-performance rocket motor igniters that are intended for use in vacuum space.